A Siemens method is known as a method for manufacturing polycrystalline silicon that is a raw material of monocrystalline silicon for manufacturing semiconductors or silicon for manufacturing solar cells. The Siemens method is a method of bringing a material gas containing chlorosilane into contact with a heated silicon core to grow polycrystalline silicon from vapor phase on a surface of the silicon core using a CVD (Chemical Vapor Deposition) method.
In growing polycrystalline silicon from vapor phase by the Siemens method, two vertical silicon cores and one horizontal silicon core are assembled into an inverted U-shape in a reactor of a vapor phase growth device, and opposite ends of the vertical silicon cores are secured to a pair of metal electrodes placed on a base plate via a pair of core holders.
Next, a current is applied from the metal electrodes to heat the silicon cores to a temperature range of 900° C. to 1200° C. in a hydrogen atmosphere, and a material gas, for example, a mixed gas of trichlorosilane and hydrogen is supplied from a gas nozzle into the reactor. Then, silicon is grown from vapor phase on the silicon core, and a polycrystalline silicon rod having a desired diameter is formed into an inverted U-shape. After the reactor is cooled, the polycrystalline silicon rod is taken out of the reactor.
In recent years, with increasing diameter of a polycrystalline silicon rod, a crack or a break easily occurs in the polycrystalline silicon rod during vapor phase growth or cooling of the polycrystalline silicon rod.
This may be because, in growing a polycrystalline silicon rod by a Siemens method, a temperature difference occurs between a center and a surface in a growing direction (radial direction) of the silicon rod during or after vapor phase growth, and this causes stress by thermal expansion or contraction of the polycrystalline silicon rod.
If the polycrystalline silicon rod breaks and falls in the reactor, heavy metal contamination occurs due to contact with an inner wall of the reactor and metal that constitutes a base plate or a metal electrode, and also it takes time to collect the collapsed polycrystalline silicon rod and clean the base plate to significantly increase an operation cycle time, thereby significantly reducing productivity.
Various proposals have been made to prevent occurrence of such a crack or a break of a polycrystalline silicon rod.
For example, Japanese Patent Laid-Open No. 8-45847 (Patent Literature 1) proposes a mounting tool of a carrier member (core) including at least one spring element provided between a current lead portion (metal electrode) and an electrode holder (holding tool of core holder), wherein the spring element allows movement of the electrode holder with respect to the current lead portion and also absorbs the movement.
Japanese Patent Laid-Open No. 2006-16243 (Patent Literature 2) proposes that a seed holding electrode including a carbon seed holder and a metal electrode, in which the seed holder and the metal electrode are joined by fitting in a tapered shape, and a noble metal sheet is joined therebetween in a rubbing manner, is used to prevent a break of polycrystalline silicon or a carbon component used in the seed holding electrode due to thermal strain generated in a cooling step after production of polycrystalline silicon.
Japanese Patent Laid-Open No. 2006-240934 (Patent Literature 3) proposes that a carbon holder, in which ends of a silicon core are electrically connected to electrodes via conductive holders holding the silicon cores and at least one holder is slidable on an electrode surface both to left and right in a direction of a line connecting opposite ends of an inverted U-shaped silicon core, is used to reduce occurrence of cracks in a polycrystalline silicon rod.